Wurtzite gallium nitride (GaN) has great potential for high-frequency and high-power applications due to its excellent electrical and thermal transport properties. However, enhancing the performance of GaN-based power electronics relies on heavy doping. Previous studies showed that electron-phonon interactions have strong effects on the lattice thermal conductivity of GaN due to the Fröhlich interaction. Surprisingly, our investigation reveals weak effects of electron-phonon interactions on the lattice thermal conductivity of $\mathit{n}$-type GaN at ultrahigh electron concentrations and the impact of the Fröhlich interaction can be ignored. The small phonon-electron scattering rate is attributed to the limited scattering channels, quantified by the Fermi surface nesting function. In contrast, there is a significant reduction in the lattice thermal conductivity of $\mathit{p}$-type GaN at high hole concentrations due to the relatively larger Fermi surface nesting function. Meanwhile, as $\mathit{p}$-type GaN has relatively smaller electron-phonon matrix elements, the reduction in lattice thermal conductivity is still weaker than that observed in $\mathit{p}$-type silicon. Our work provides a deep understanding of thermal transport in doped GaN and the conclusions can be further extended to other wide-band-gap semiconductors, including $\beta -{\mathrm{Ga}}_2{\mathrm{O}}_3$, AlN, and ZnO.

To realize room-temperature ferromagnetic semiconductors is still a challenge in spintronics. Recent experiments have obtained two-dimensional (2D) room-temperature ferromagnetic metals, such as monolayer ${\mathrm{MnSe}}_2$. In this paper, we proposed a way to obtain room-temperature ferromagnetic semiconductors through metal-semiconductor transition. By the density-functional theory calculations, a room-temperature ferromagnetic semiconductor is obtained in monolayer ${\mathrm{MnSe}}_2$ with a few-percent tensile strain, where a metal-semiconductor transition occurs with 2.2% tensile strain. The tensile strains raise the energy of $d$ orbitals of Mn atoms and $p$ orbitals of Se atoms near the Fermi level, making the Fermi-level sets in the energy gap of bonding and antibonding states of these $p$ and $d$ orbitals, and opening a small band gap. The room-temperature ferromagnetic semiconductors are also obtained in the heterostructures ${\mathrm{MnSe}}_2$/X (X = ${\mathrm{Al}}_2{\mathrm{Se}}_3$, GaSe, SiH, and GaP), where metal-semiconductor transition happens due to the tensile strains by interface of heterostructures. In addition, a large magneto-optical Kerr effect (MOKE) is obtained in monolayer ${\mathrm{MnSe}}_2$ with tensile strain and ${\mathrm{MnSe}}_2$-based heterostructures. Our theoretical results pave a way to obtain room-temperature magnetic semiconductors from experimentally obtained 2D room-temperature ferromagnetic metals through metal-semiconductor transitions.

In the fluctuation regime that precedes the onset of pairing in multicomponent superconductors, such as nematic and chiral superconductors, the normal state is generally unstable toward the formation of charge-$4e$ order—an exotic quantum state in which electrons form coherent quartets rather than Cooper pairs. However, charge-$4e$ order is often suppressed by other competing composite orders, such as nematics. Importantly, the formation of nematic domains is unavoidable due to the long-range strains generated, leading to one-dimensional regions where the competing nematic order is suppressed. Here, we employ a real-space variational approach to demonstrate that, in such nematic domain walls, charge-$4e$ order is locally condensed via a vestigial-order mechanism. We explore the experimental manifestations of this effect and discuss materials in which it can be potentially observed.

Altermagnets exhibit a large electron spin splitting which can be understood as a result of strong coupling between itinerant electrons and localized spins. We consider superconductivity due to electron-magnon scattering, using strong-coupling Eliashberg theory to capture many-body effects that are not covered by a weak-coupling approach. The characteristic band structure of altermagnets puts significant constraints on the spin structure of electron scattering on the Fermi surface. We emphasize the role of spin-preserving, double-magnon scattering processes compared to conventional spin-flip processes involving a single magnon. Then, we derive the Eliashberg equations for a situation where double-magnon scattering mediates spin-polarized Cooper pairs, while both double-magnon and single-magnon scatterings contribute to many-body effects. These many-body effects impact superconducting properties in a way that differs significantly from systems where conventional spin-flip processes mediate superconductivity. To highlight the role of $d$-wave magnetism on superconductivity in altermagnets, we compare our results to those found in ferromagnetic half-metals and conventional antiferromagnetic metals.

The coupling between two ferromagnets separated by a superconductor has been mostly investigated for the case of Cooper pairs with spin-singlet symmetry. Here, we consider a spin-triplet superconductor with chiral pairing. By full self-consistent analysis of the spatial dependent superconducting order parameter, we determine the magnetic ground state of the superconducting spin valve. The study is performed by investigating the role of the orientation and strength of the magnetization in the ferromagnets including spin-valve asymmetries in the magnetic configurations. Due to the nonvanishing angular momentum of the spin-triplet Cooper pairs, we demonstrate that the induced magnetic coupling has an anisotropic character and a structure that can favor collinear or noncollinear magnetic orientations, thus mimicking a magnetic interaction of the Heisenberg or Dzyaloshinskii-Moriya type, respectively. We investigate the role of the physical parameters controlling the character of the magnetic exchange: the amplitude of the magnetization in the ferromagnets, and the length of the superconducting spacer in the spin valve. Our study demonstrates that spin-triplet superconductors can be employed to devise anisotropic magnetic exchange and to allow for transitions in the spin-valve state from a collinear to noncollinear magnetic configuration.

The attainment of high-quality wave concentration and manipulation has always been considered as state-of-the-art technology, especially for integrated photonics and phononics. However, the prevention of energy loss caused by backscattering or imperfections remains a grand challenge. With the development of the topological phase of matter, the emergence of topological insulators that support robust conductive edge states but insulating bulk waves provides a possible solution. Nevertheless, the existing topological insulators can only achieve wave manipulation in two-dimensional (2D) models along specific hinges. To achieve lossless waveguiding in three-dimensional space, an acoustic topological insulator with three degrees of freedom is established. The theoretical dispersion relation is analyzed by introducing an equivalent electric circuit system. The topological states, including point corner states, one-dimensional hinge states, and 2D surface states are realized by tweaking the intra- and intercell couplings. Abundant wave propagation behaviors such as surface-restricted, edge-restricted, and corner-restricted wave transportation are respectively achieved in the first-, second-, and third-order topological insulators. The twisted 3D path waveguiding without significant energy leaking into surface and bulk is finally demonstrated. This sound transportation phenomenon may provide a paradigm and design idea for integrated acoustic devices with unconventional functions.

The newly discovered altermagnets are unconventional collinear compensated magnetic systems, exhibiting even ($d, g$, or $i$ wave) spin-polarization order in the band structure, setting them apart from conventional collinear ferromagnets and antiferromagnets. Altermagnets offer advantages of spin-polarized current akin to ferromagnets, and THz functionalities similar to antiferromagnets, while introducing new effects like spin-splitter currents. A key challenge for future applications and functionalization of altermagnets is to demonstrate controlled transitioning to the altermagnetic phase from other conventional phases in a single material. Here we prove a viable path toward overcoming this challenge through a strain-induced transition from an antiferromagnetic to an altermagnetic phase in ${\mathrm{ReO}}_2$. Combining spin group symmetry analysis and ab initio calculations, we demonstrate that under compressive strain ${\mathrm{ReO}}_2$ undergoes such transition, lifting the Kramers degeneracy of the band structure of the antiferromagnetic phase in the nonrelativistic regime. In addition, we show that this magnetic transition is accompanied by a metal-insulator transition, and calculate the distinct spin-polarized spectral functions of the two phases, which can be detected in angle-resolved photoemission spectroscopy experiments.

The interlayer Dzyaloshinskii-Moriya interaction (IL-DMI) has recently emerged as an ingredient promoting chiral orthogonal coupling between adjacent magnetic layers in multilayered systems. IL-DMI offers an additional tuning knob to engineer the magnetic behavior in spintronic devices, which could be useful for nonvolatile logic and memory technologies. Here, we systematically study, via first-principles calculations and the three-site Fert-Lévy model, the spacer thickness dependence of the IL-DMI between an out-of-plane ferrimagnet TbCo and an in-plane ferromagnet Co through Pt, Ir, Pd, and Ru. We observed a damped oscillatory behavior with increasing spacer thickness in all cases with characteristic amplitude and periodicity. Furthermore, we established a direct correlation between the IL-DMI and density of states of bottom and top Co atoms, dominated by the spacer thickness, which is attributed to a hybridization of electronic orbitals. Based on this compelling evidence, we propose that the electronic orbital hybridization contributes to the microscopic origin of the IL-DMI metallic magnetic multilayers. We anticipate that our results will provide insights into the understanding and precise control of IL-DMI in a wide range of materials and spintronic device concepts.

High-symmetry rocksalt type tetragonal CuO (T-CuO) does not exist in bulk but can be synthesized via thin film epitaxy limited to a few unit cells (3, 4) thick and above which it relaxes to its bulk tenorite structure. Direct probe into magnetic properties of T-CuO layer has been a challenge because of its ultrathin limit. Here, we demonstrate the interfacial magnetic coupling between ultrathin T-CuO and ferromagnetic (${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$) layers in an epitaxial $\mathrm{CuO}\text{/}{\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ bilayer grown on (001)-oriented ${\mathrm{SrTiO}}_3$. We observe a positive exchange bias shift of $\sim 30$ Oe at 2 K in $\mathrm{CuO}\text{/}{\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ bilayer. The observation of positive exchange bias indicates that there exists antiferromagnetic exchange coupling between Mn and Cu moments at the interface. Notably, the exchange bias vanishes at 5 K and it is discussed in view of the proposed spin structure revealed from low-energy muon spin rotation and x-ray magnetic circular dichroism study [Phys. Rev. B 103, 224429 (2021)]. Furthermore, an enhanced Gilbert damping, linewidth broadening and larger inhomogeneous $4\pi M_{\mathrm{eff}}$ value from in-plane ferromagnetic resonance measurements, are the direct consequence of antiferromagnetic exchange coupling at the $\mathrm{CuO}\text{/}{\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ interface. Combining both static and dynamic magnetic characterization, we establish an understanding of interfacial exchange coupling in $\mathrm{CuO}\text{/}{\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ bilayer.

$4H_b\text{−}\mathrm{Ta}{\mathrm{S}}_2$ is a naturally formed quasi-two-dimensional heterojunction material composed of alternating monolayers of insulating ($1T$-) and superconducting ($1H$-) $\mathrm{Ta}{\mathrm{S}}_2$. We report on a comprehensive high-pressure study on the interplay between charge-density wave (CDW) and superconductivity (SC) in $4H_b\text{−}\mathrm{Ta}{\mathrm{S}}_2$. The results uncover a dome-shaped strong-coupling superconductor on the border of a 3 × 3 commensurate CDW at $P_{\mathrm{c}1}\approx 2.0$ GPa: (1) nearly one order enhancement of upper critical field $B_{c2}$(0); (2) the derived $2{\mathrm{\Delta }}_0/k_{\mathrm{B}}{T}_c$ beyond the BCS theory, decaying to a conventional one above 4.5 GPa; (3) the exponent of normal-state resistivity $n\approx 1.50$ and triply enhanced electronic effective mass. Under pressure, a third CDW emerging from 2.0 GPa is related to the original two CDWs, then disappears above 11.5 GPa. The temperature dependence of $B_{c2}\left(T\right)$ collapses into a universal curve and the comparison of $B_{c2}\left(T\right)$ to a polar-state function in $4H_b\text{−}\mathrm{Ta}{\mathrm{S}}_2$. Theoretical calculations proposed the stronger interlayer coupling and band hybridization responsible for strong-coupling SC; the suppression of new CDWs is related to band inversions between the $1T$-Ta-$d_{xy}$ (GM1+) and S-$p_z$ bands (GM2-) at the $\mathrm{\Gamma }A$ point; above 5.0 GPa, the coexisting weak-coupling SC and linear magnetoresistance can be attributed to the formation of electronic bands along the $M-K$ lines and the $\mathrm{\Gamma }A$ point. Our discovery provides an excellent example to demonstrate the interplay of the strong-coupling superconducting state and topological electronic state in van der Waals heterojunctions.

Schmid transition was introduced first as a superconductor-insulator transition in the zero-frequency response of a shunted Josephson junction in equilibrium at zero temperature. As it is typical for a quantum impurity problem, at finite frequencies the transition is broadened to a crossover. Modern attempts to find Schmid transition rely on finite-frequency measurements of a quantum circuit. We predict the frequency dependence of the admittance and reflection phase shift for a high-impedance transmission line terminated by a Josephson junction for a wide variety of devices, from a charge qubit to a transmon. Our results identify the circuit parameters needed for the experimental observation of universal scaling of the responses with frequency. On the insulating side of the transition, the full crossover from weak to strong coupling can only be observed in a transmon. On the contrary, observation of such crossover on the superconducting side of the transition is possible only with the charge qubit. The frequency dependence gets weaker and vanishes upon approaching the Schmid transition from either side.

In gapped bilayer graphene, similarly to conventional semiconductors, Coulomb impurities (such as nitrogen donors) may determine the activation energy of its conductivity and provide low-temperature hopping conductivity. However, in spite of the importance of Coulomb impurities, nothing is known about their electron binding energy $E_b$ in the presence of gates. To close this gap, we study numerically the electron binding energy $E_b$ of a singly charged donor in BN-enveloped bilayer graphene with the top and bottom gates at distance $d$ and gate-tunable gap $2\mathrm{\Delta }$. We show that for $10<d<200\mathrm{nm}$ and $1<\mathrm{\Delta }<70\mathrm{meV}$ the ratio $E_b/\mathrm{\Delta }$ changes from 0.4 to 1.4. The ratio $E_b/\mathrm{\Delta }$ stays so close to unity because of the dominating role of the bilayer polarization screening which reduces the Coulomb potential well depth to values $\sim \mathrm{\Delta }$. Still, the ratio $E_b/\mathrm{\Delta }$ somewhat decreases with growing $\mathrm{\Delta }$, faster at small $\mathrm{\Delta }$ and slower at large $\mathrm{\Delta }$. On the other hand, $E_b/\mathrm{\Delta }$ weakly grows with $d$, again faster at small $\mathrm{\Delta }$ and slower at large $\mathrm{\Delta }$. We also studied the effect of trigonal warping and found only a small reduction of $E_b/\mathrm{\Delta }$.

As an alternative platform to unravel some of the perplexing characteristics of strontium ruthenate, we propose to study the isostructural and more correlated material strontium ferrite. Using density functional theory combined with dynamical mean-field theory, we attribute the experimentally observed insulating behavior at zero pressure to strong local electronic correlations generated by Mott and Hund's physics. At high pressure, our simulations reproduce the reported insulator-to-metal transition around 18 GPa. Along with distinctive features of a Hund metal, the resulting metallic state is found to display an electronic structure analogous to that of strontium ruthenate, suggesting that it could exhibit similar low-energy properties.

The spin Chern topological phases are more natural in solid-state systems and are thought to exist in two or three dimensions. To date, there is no evidence for the existence of spin Chern topological phase in non-integer dimension. Fractal offers a platform for exploring novel topological phases and phenomena in noninteger dimension. Here, based on a phononic fractal lattice, we experimentally demonstrate the presence of the spin Chern phase in noninteger dimension. We find that the spin Chern phase is compressed in the fractal lattice compared to the crystal lattice. We also highlight the robustness and unidirectionality of spin-polarized topologically protected edge states even the momentum space is ill defined. Interestingly, sound travels faster at the boundaries of the fractal lattice than in crystal lattice. Abundant spin-polarized edge states and increased velocities not only may inspire further study in other noninteger dimensional systems, but also provide an opportunity for the design of multichannel on-chip communication devices.

Conduction of spin currents in disordered insulating antiferromagnets has recently been at the center of scientific debate with both long-range spin transport or no spin transport at all observed experimentally. In this study, ferromagnetic resonance has been used to probe the transmission of ac spin current through thin amorphous yttrium iron garnet (YIG) layers. The spin current is found to be mediated by evanescent spin waves with a penetration length four times larger than that of previous studies of amorphous YIG, even exceeding the spin penetration length of crystalline NiO.

Previous research has demonstrated the spin-orbit torque (SOT) switching of $\mathrm{M}{\mathrm{n}}_3\mathrm{Sn}$ in configuration I, where the spin polarization σ resides within the kagome plane. However, this configuration has yielded several unexpected outcomes, giving rise to debates concerning the fundamental physics governing the switching process. Alternatively, in configuration II, σ is perpendicular to the kagome plane, which bears greater resemblance to the ferromagnetic system. In this study, we show successful SOT switching of $\mathrm{M}{\mathrm{n}}_3\mathrm{Sn}$ in configuration II, demonstrating behaviors more akin to ferromagnets, e.g., the critical switching current density ($J_{\mathrm{crit}}$) and external field ($H_{\mathrm{ext}}$) are in the order of ${10}^{10}\mathrm{A}/{\mathrm{m}}^2$ and tens of Oersted, respectively. The switching result is also independent of the initial state. We further show that the distinctive spin structure of $\mathrm{M}{\mathrm{n}}_3\mathrm{Sn}$ leads to unique switching characteristics, including $J_{\mathrm{crit}}$ increasing linearly with $H_{\mathrm{ext}}$ and the opposite switching polarity to ferromagnetism. A switching phase diagram is further provided as a guideline for experimental demonstrations, offering a clear physical picture for the observed phenomena.

It is well known that electronic states in graphene with a uniform Rashba spin-orbit interaction and uniform magnetization, e.g., due to exchange coupling to a magnetic substrate, display an energy gap around the Dirac $K$ and $K^{\prime }$ points. When the magnetization of graphene is nonuniform and forms a magnetic domain wall, electronic states localized at the wall emerge in the energy gap. In this paper we show that similar localized electronic states appear in the gap when the graphene is uniformly magnetized, while a domain wall appears in the Rashba spin-orbit interaction (i.e., opposite signs of the Rashba parameter on both sides of the wall). These electronic states propagate along the wall and are localized exponentially at the Rashba domain wall. They form narrow and nearly parabolic (at small wave vectors) bands, with relatively large effective electron mass. However, contrary to the magnetic domain wall, these states do not close the energy gap. We also consider the situation when the magnetic domain wall coexists with the Rashba domain wall, and both walls are localized at the same position. Electronic states due to the interplay of both domain walls are determined analytically and it is shown that the electronic states localized at the walls close the gap when a magnetic domain wall (symmetric or asymmetric) exists, independently of the Rashba parameter behavior.

Fullerene solids doped with alkali metals (${\mathrm{A}}_3{\mathrm{C}}_{60}$, A = K, Rb, Cs) exhibit a superconducting transition temperature $T_c$ as high as 40 K, and their unconventional superconducting properties have been a subject of debate. With the application of high pressure on ${\mathrm{K}}_3{\mathrm{C}}_{60}$ and ${\mathrm{Rb}}_3{\mathrm{C}}_{60}$, experiments demonstrate the decrease of $T_c$. In this paper, we focus on ${\mathrm{K}}_3{\mathrm{C}}_{60}$ and derive the structure of ${\mathrm{K}}_3{\mathrm{C}}_{60}$ under different pressures based on first-principles calculations, exploring the trends of Coulomb interactions at various pressures. By utilizing the maximally localized Wannier function approach, constrained density functional perturbation theory, and constrained random phase approximation, we construct a microscopic low-energy model near the Fermi level. Our results strongly indicate that, in the ${\mathrm{K}}_3{\mathrm{C}}_{60}$ system, as pressure increases, the effect of phonons is the key to intraorbital electron pairing. The phonon-driven superconducting mechanism is dominant at high pressure.

We consider a one-dimensional system of noninteracting fermions featuring both boundary driving and continuous monitoring of the bulk particle density. Due to the measurements, the expectation values of the local density and current operators are random variables whose average behavior is described by a well-studied Lindblad master equation. By means of exact numerical computations, we go beyond the averaged dynamics and study their full probability distribution functions, focusing on the late-time stationary regime. We find that contrary to the averaged values, the spatial profiles of the median density and current are nontrivial, exhibiting qualitative differences as a function of the monitoring strength. At weak monitoring, the medians are close to the means, displaying diffusive spatial profiles. At strong monitoring, we find that the median density and current develop a domain-wall and single-peak profile, respectively, which are suggestive of a Zeno-like localization in typical quantum trajectories. While we are not able to identify a sharp phase transition as a function of the monitoring rate, our work highlights the usefulness of characterizing typical behavior beyond the averaged values in the context of monitored many-body quantum dynamics.